Temperature and Precipitation Projections

Global average temperatures are projected to rise over this century and beyond, causing continued changes in all components of the climate system. Temperature increases will vary regionally and seasonally; for example, temperature increases at polar latitudes are expected to be greater than increases near the equator (USGCRP 2014 Ch. 2). Part of this future warming is inevitable due to the long-lived greenhouse gases that are already present in Earth’s atmosphere.

Climate varies without human influence, and this natural variation is a backdrop for the human-caused climate change occurring now. These patterns hold important lessons for understanding the magnitude and scope of current and future climate changes.

The Greenhouse Effect

The physical mechanisms that cause greenhouse gases to warm the planet, commonly known as the greenhouse effect, are well understood and were scientifically demonstrated beginning in the mid-1800s (Tyndal 1861). Of the solar energy that is directed toward Earth, about 30% is reflected back to space by clouds, dust, and haze (Ramanathan & Feng 2009). The remaining 70% is absorbed by the atmosphere and the Earth’s surface.

Global Warming and Rising CO2

Global average surface temperatures have increased markedly over the last century (Figure 1). Humans have been measuring temperature directly since the mid- 1800’s; these measurements show that temperature increased by 1.53°F (0.85°C ) between 1880 and 2012, and that the rate of warming is increasing (IPCC 2013 Ch. 2). With the exception of 1998, the 10 warmest years in the 134-year record all have occurred since 2000, with 2010 and 2005 ranking as the warmest years on record (NASA GIS 2014).